Meta-Analysis of Neurochemical Changes Estimated via Magnetic Resonance Spectroscopy in Mild Cognitive Impairment and Alzheimer's Disease

The changes of neurochemicals in mild cognitive impairment (MCI) and Alzheimer's disease (AD) patients has been observed via magnetic resonance spectroscopy in several studies. However, whether it exists the consistent pattern of changes of neurochemicals in the encephalic region during the progression of MCI to AD were still not clear. The study performed meta-analysis to investigate the patterns of neurochemical changes in the encephalic region in the progress of AD. We searched the PubMed, Embase, Cochrane Library, and Web of Science databases, and finally included 63 studies comprising 1,086 MCI patients, 1,256 AD patients, and 1,907 healthy controls. It showed that during the progression from MCI to AD, N-acetyl aspartate (NAA) decreased continuously in the posterior cingulate (PC) (SMD: −0.42 [95% CI: −0.62 to −0.21], z = −3.89, P < 0.05), NAA/Cr (creatine) was consistently reduced in PC (SMD: −0.58 [95% CI: −0.86 to −0.30], z = −4.06, P < 0.05) and hippocampus (SMD: −0.65 [95% CI: −1.11 to −0.12], z = −2.44, P < 0.05), while myo-inositol (mI) (SMD: 0.44 [95% CI: 0.26–0.61], z = 4.97, P < 0.05) and mI/Cr (SMD: 0.43 [95% CI: 0.17–0.68], z = 3.30, P < 0.05) were raised in PC. Furthermore, these results were further verified by a sustained decrease in the NAA/mI of PC (SMD: −0.94 [95% CI: −1.24 to −0.65], z = −6.26, P < 0.05). Therefore, the levels of NAA and mI were associated with the cognitive decline and might be used as potentially biomarkers to predict the possible progression from MCI to AD. Systematic Review Registration: https://www.crd.york.ac.uk/PROSPERO/, identifier: CRD42020200308.


INTRODUCTION
Alzheimer's disease (AD) is a neurodegenerative disease with agerelated progressive cognitive impairment (Scheltens et al., 2016). According to Alzheimer's Disease International, the number of AD patients worldwide was about 50 million in 2018, which would be tripled by 2050 (Scheltens et al., 2021). Early detection and identification of the preclinical AD stage has been thought to be extremely important for slowing down the disease progression of AD. As the prodromal state of AD, there are about 31.5% of mild cognitive impairment (MCI) patients who will be converted to AD within 5 years (Ward et al., 2013). Therefore, exploring the potential biomarkers from MCI to AD is critical for early identification and developing evidence-based interventions of the condition.
The aggregation of amyloid-β (Aβ) in Aβ-pleated and the accumulation of tau in neurofibrillary tangles (NFT) is thought to be the key pathological features of AD (Holtzman et al., 2011). The cerebrospinal fluid (CSF) biomarkers Aβ 42, total tau, and phosphorylated tau are found to be sensitive and common biomarkers that can effectively reflect the typical pathological features of AD (Palmqvist et al., 2016;Sun et al., 2020). The main detection methods of these biomarkers are imaging examination such as 11 C-labeled Pittsburgh compound-B(PIB)-positron emission tomography (PET) or biological fluids examination, especially from CSF (Mattsson-Carlgren et al., 2020). However, the abnormalities of Aβ 42, T-tau, and P-tau may lag behind cognitive impairment (Jack et al., 2013). Moreover, some changes of biomarkers are found at a stage which the basic neuropathological examination has reached an advanced and irreversible state and needed to be tested and verified by autopsy and histopathology. As a result, it is urgent to identify sensitive and specific biomarkers and detection methods for facilitating early detection and effective treatment of AD.
With the advantages of non-invasiveness, higher sensitivity, and without any radiation, magnetic resonance spectroscopy (MRS) has been widely used to assess the changes of neurochemicals in specific brain tissues in MCI and AD. Increasing evidence suggests a link between the incidence and progression of AD and metabolic dysfunction. Studies have found that neurochemicals, including N-acetyl aspartate (NAA), choline (Cho), creatine (Cr), myo-inositol (mI), and glutamate and glutamine (Glx), have abnormal metabolic changes in the pathological process of AD. NAA is a specific metabolite of the nervous system, which is synthesized by aspartic acid and acetyl-CoA in neuronal mitochondria, and is highly expressed in neuronal mitochondria. It is widely considered as a specific indicator of neuronal activity. Studies have shown that the level of NAA is closely associated with cognitive dysfunction, especially memory impairment (Jessen et al., 2000). Moreover, the autopsy results showed that the level of NAA was decreased in AD patients. Cho signal is related to cell membrane phospholipid metabolism, which mainly reflects the damage of cholinergic neurons. When the cell membrane is destroyed, the level of Cho will show an increasing trend. In addition, Cho has a close relationship with learning, recall, and other cognitive abilities (Khomenko et al., 2019). The level of Cr in vivo is relatively stable, and is closely related to energy metabolism, maintaining ATP level in cells, but the content is reduced in the late stage of AD. Myo-inositol has a role in the second messenger cycle and is regarded as a marker of glial cells. Studies have found that mI level increased in the hippocampus in MCI but decreased in the late period of AD (Voevodskaya et al., 2016). Glx is a key amino acid in the brain and studies showed that a decrease of Glx and cognitive impairment always occurred simultaneously . Therefore, research on the changes of these neurochemicals in the brain may be helpful for the early diagnosis of MCI and AD.
In recent years, several studies have applied MRS to detect the metabolic changes of neurochemicals in the brain of MCI and AD patients to predict the progress of the condition. However, the results were various. The ratio of NAA/mI is often used to distinguish AD from normal people, and the sensitivity was as high as 83% (Kantarci et al., 2000). Interestingly, one study suggested that the NAA/mI in the posterior cingulate (PC) of MCI patients decreased (Mitolo et al., 2019), while another study found that the NAA/mI showed an increase trend in the same brain region (García et al., 2008). Previous studies have found that the ratio of NAA/Cr in the medial temporal lobe (MTL) is increased in AD patients, indicating neuron damage in the brain (Jessen et al., 2009). On the contrary, the level decreased markedly in the MTL region of AD patients in another study (Chao et al., 2005). To investigate whether there would be a consistent pattern of changes of neurochemicals in the encephalic region in the progress of AD, a meta-analysis was conducted. The goal was to identify the changes of abnormal neurochemicals in typical brain regions from MCI to AD.  region was reported. (4) Specifications for spectrum acquisition were reported. The exclusion criteria were as follows: (1) studies were published in languages other than English. (2) Original data could not be extracted, or the full text could not be obtained.
(3) Duplicate or similar data published research. (4) The subjects were animals. (5) The subjects were taking drugs, had other significant medical conditions or substance abuse that could interfere with cognitive functioning.

Data Extraction
After applying the inclusion and exclusion criteria, we finally identified 63 articles and extracted the following characteristics for meta-analysis: the interested brain regions and the corresponding metabolites ratios and concentrations, the field strength, repetition time/echo time (TR/TE), and other characteristics which are shown in Table 1. Meanwhile, we also extracted standard deviations (SD) or standard error of mean (SEM) or median, as our main results.

Statistical Analyses
Stata 16.0 (Stata Corp) software was used to perform all statistical analysis. The sample size, mean value, and SD were used to generate the effective sizes, and when the mean ± SEM or median was provided, we converted it into mean ± SD for meta-analysis. Then, we calculated the standardized mean difference (SMD) and 95% CI and drew a forest map to compare the relationship between the metabolites' concentrations or metabolites' ratios between the healthy control group and AD patients, the healthy control group and MCI patients, and AD patients and MCI patients. We used the Q-test and I 2 index to evaluate heterogeneity. The statistical significance of the Qtest was set as P < 0.1, and heterogeneity was assessed by I 2 index, with 25, 50, and 75%, indicating that the heterogeneity was low, medium, and high (Higgins et al., 2003). For the statistical model, we first chose fixed effects model with the method of inverse-variance for analysis. If the heterogeneity of the results is greater, we used the random effect model with the method of Dersimonian and Laird for analysis. All statistical significances were set at P < 0.05 and P < 0.1 was regarded as a trend.
The meta-analysis comprised of a total of 3,271 subjects, with 1,086 MCI patients, 1,256 AD patients, and 1,907 healthy controls. The following regions were investigated: hippocampus (including MTL), PC, temporal lobe, occipital regions, paratrigonal white matter area, temporo-parietal lobe, parietal lobe, frontal lobe (gray and white matter area), and

References
Selection   *means that this study awarded one score on this question.
Frontiers in Aging Neuroscience | www.frontiersin.org anterior cingulate. The key characteristics of the studies are shown in Table 1 among which 6 studies were classified as high quality and 57 studies were classified as medium quality ( Table 2).

Metabolite Ratios
We compared the ratios of five metabolites, extracting data from 243 MCI patients and 282 healthy control subjects in 10 studies (Ackl et al., 2005;Franczak et al., 2007;Wang et al., 2009Wang et al., , 2012Zhang et al., 2009;Seo et al., 2012;Liu et al., 2013;Targosz-Gajniak et al., 2013;Zhu et al., 2015;Huang et al., 2017). The results showed that four metabolites' ratios (NAA/Cr, Cho/Cr, mI/Cr, and mI/NAA) were significantly different in MCI and healthy control subjects, but there was no significant difference in Glx/Cr ( In addition, 11 studies (Jessen et al., 2000(Jessen et al., , 2005(Jessen et al., , 2009Block et al., 2002;Ackl et al., 2005;Chao et al., 2005;Wang et al., 2009Wang et al., , 2012Zhang et al., 2009;Shiino et al., 2012;Huang et al., 2017) were combined to compare the ratios of NAA/Cr, Cho/Cr, mI/Cr, and mI/NAA in 600 AD patients and 428 healthy control subjects. The results showed that four metabolites' ratios (NAA/Cr, Cho/Cr, mI/Cr, and mI/NAA) were significantly different in AD and healthy control subjects. For the comparisons between AD and controls, NAA/Cr (SMD: Next, we performed a meta-analysis to compare the ratios of NAA/Cr, between 155 AD patients and 130 MCI patients (Ackl et al., 2005;Wang et al., 2009Wang et al., , 2012Zhang et al., 2009;Huang et al., 2017). The results demonstrated significant difference. NAA/Cr (SMD: −0.62 [95% CI: −1.11 to −0.12], z = −2.44, P < 0.05, Figures 2C, 9) in the MCI group were significantly higher than that in the AD patients. Four studies (Ackl et al., 2005;Wang et al., 2009Wang et al., , 2012Zhang et al., 2009)  Moreover, studies Seo et al., 2012) longitudinally tracked the metabolite differences in the hippocampus between MCI-converter and MCI-stable patients. Above all, according to the results of meta-analysis of AD and MCI, the ratios of NAA/Cr and Cho/Cr are both decreased in AD and MCI patients. Notably, the decrease was more obvious in AD patients. In addition, mI/Cr and mI/NAA ratios were seen to increase faster in AD patients, compared to subjects who converted to MCI and cognitively normal elderly.
In conclusion, based on the analysis of AD and MCI, 3 metabolites' concentrations (NAA, Cr, and Cho) were found to be lower in AD patients as compared to MCI patients and healthy control subjects.
Two studies were extracted to compare the ratios in 25 MCI-converter patients and 37 MCI-stable patients (Kantarci et al., 2007;Seo et al., 2012 Taken together, these results suggest that the ratios of NAA/Cr and NAA/mI were reduced in AD patients as compared to MCI patients and healthy controls. However, in the posterior cingulate, mI/NAA and Glx/Cr decreased remarkably compared to that of AD patients.
Briefly, according to the results of meta-analysis of AD and MCI, the concentration of NAA was decreased in AD and MCI patients, especially in AD patients. In addition, mI concentration was seen to increase faster in AD patients, compared to subjects who converted to MCI and cognitively normal elderly.

Metabolite Ratios
We finally identified 3 studies with a total sample size of 326 (187 MCI patients and 139 healthy controls) to compare the ratio of NAA/Cr in the parietal lobe. The analysis revealed that there was no significant difference observed between the two groups (SMD: 0.02 [95% CI: −0.20 to 0.24], z = 0.16, P > 0.1, Supplementary Table 1).

Meta-Analysis of the Occipital Lobe
There were 5 studies (Block et al., 2002;Azevedo et al., 2008;Watanabe et al., 2010;Graff-Radford et al., 2014;Marjańska et al., 2019) with a total sample size of 481 (195 AD patients and 286 healthy controls) included for meta-analysis to investigate the ratio of metabolites in the occipital lobe. Moreover, there were 3 more studies (Modrego et al., 2005Seo et al., 2012) longitudinally tracking the metabolite differences between MCI-converter and MCI-stable patients.
a comparison in the changes between 83 AD patients and 70 healthy control subjects, and 3 studies (Lim et al., 2012;Guo et al., 2016;Huang et al., 2017) were conducted to observe the differences of metabolites in 83 AD patients and 66 MCI patients.

Metabolite Ratios
First, we identified 3 studies (Lim et al., 2012;Guo et al., 2016;Huang et al., 2017) with a total sample size of 136 (66 MCI patients and 70 healthy controls) to compare the ratio of NAA/Cr in the anterior cingulate. The analysis showed that there was no difference between the two groups (SMD: −0.20 [95% CI: −0.68 to 0.29], z = −0.80, P > 0.1, Supplementary Table 1).

Meta-Analysis of the Temporo-Parietal Lobe
Four studies (Ernst et al., 1997;Fernández et al., 2005;Rami et al., 2007;Zhang et al., 2009) investigated the temporo-parietal lobe including 80 AD patients and 71 healthy control subjects to compare the metabolites between the two groups.

Metabolite Ratios
We compared the ratios of metabolites, extracting data from 157 AD patients and 110 healthy control subjects in 4 studies (Ernst et al., 1997;Fernández et al., 2005;Rami et al., 2007;Zhang et al., 2009). The results showed that two metabolites' ratios (NAA/Cr, mI/Cr) were significantly different in AD and healthy control subjects. NAA/Cr was significantly decreased in the temporo-parietal lobe of AD patients (SMD:

Meta-Analysis of the Frontal Region
Four studies (Parnetti et al., 1997;Chao et al., 2005;Zhu et al., 2006;Siger et al., 2009) with a total sample size of 218 (109 AD patients and 109 healthy controls) were included for metaanalysis to investigate the ratio of metabolites in the frontal region. Specifically, 3 studies (Parnetti et al., 1997;Zhu et al., 2006;Siger et al., 2009) compared the differences in metabolites between 61 AD patients and 61 healthy control subjects in the frontal WM, and 3 studies (Chao et al., 2005;Zhu et al., 2006;Siger et al., 2009) compared the differences in metabolites between 96 AD patients and 102 healthy control subjects in the frontal GM.

Meta-Analysis of Paratrigonal White Matter
Three studies (Catani et al., 2001;Metastasio et al., 2006;Yang et al., 2012) reported data from paratrigonal white matter including 89 MCI patients and 177 healthy control subjects to compare the metabolites between the two groups.

Metabolite Ratios
We compared the ratios of metabolites, extracting data from 89 MCI patients and 177 healthy control subjects in 3 studies (Catani et al., 2001;Metastasio et al., 2006;Yang et al., 2012). The results showed that two metabolites' ratios (NAA/Cr, mI/Cr) were significantly different between the two groups, and there was no significant difference in the ratio of Cho/Cr (SMD: 0.00 [95% CI: −0.26 to 0.25], z = −0.01, P > 0.1, Supplementary Table 1)

DISCUSSION
To investigate the changes of neurochemicals estimated by 1 H-MRS in brain regions with the progression of AD, we conducted a comprehensive meta-analysis including 63 studies with 3,271 subjects. The results showed that: (1) compared with MCI patients, the ratio of NAA/Cr in the hippocampus of AD patients decreased significantly; the ratios of NAA/Cr, NAA/mI, and the concentration of NAA in PC decreased significantly, whereas the ratios of mI/Cr and mI/NAA increased markedly. NAA and mI were considered as potential biomarkers for monitoring the progression from MCI to AD and early diagnosis of AD; (2) the metabolite difference of neurochemicals between MCI and AD was systematically analyzed and has found that the concentration of Glx in PC was different between MCI and AD patients, with an increase in AD but no changes in MCI groups. Therefore, Glx was crucial in differentiating MCI from AD, and was regarded as a potential marker to predict AD.

Metabolic Changes of Neurochemicals During the Progression From MCI to AD
NAA is an important indicator of neuronal metabolism and plays a number of roles, which includes maintaining fluid balance in the brain, providing acetic acid salt for the synthesis of myelin in oligodendrocytes, and providing energy for the glutamylation of amino acid in neuronal mitochondria. Also, NAA is considered as a biomarker of neuronal function and density, as it can reflect the density and functional state of neurons and axons. Moreover, the concentration of NAA in the cortex can provide information about the growth of neurons. In addition, its concentration in white matter can reflect the development of axons. Due to NAA being located at the synaptic terminal, dendrites, and neuronal somata, its concentration may reflect the level of synapses and the ratio of NAA/Cr reflects the integrity of synapses (Onwordi et al., 2021). Our meta-analysis showed that during the pathological development from MCI to AD, the ratio of NAA/Cr in the hippocampus and PC, and the concentration of NAA in PC decreased dramatically. Meanwhile, the level of NAA/Cr in AD patients was decreased than that in the healthy subjects in the other brain lobes, such as the parietal lobe, the frontal lobe, the temporal lobe, and the temporo-parietal lobe. Previous studies showed that the hippocampus and PC were crucial brain regions that are differentially affected by neuropathological changes in AD patients (Silverman et al., 1997). Hippocampal atrophy is one of the pathological and radiological signs of AD, and the resting state functional magnetic resonance imaging and 18 Fdeoxyglucose PET demonstrated that hippocampal dysfunction is closely related to the cognitive impairment in AD patients (Yan et al., 2020). The PC plays a crucial role in the default mode network of the brain, and previous studies demonstrated that the functional connections between the PC and the hippocampus have also been weakened during cognitive impairment (Huang et al., 2002). Studies have reported that the atrophy of the hippocampus was closely related to the loss of neurons and the number of neurons had a close connection with NAA (Shiino et al., 2012). The results of this meta-analysis suggested that with the pathological development of AD, the neurons in the hippocampus and the PC were gradually damaged. And the formation of Aβ and NFT in the brain of AD patients may lead to the decrease of NAA and the gradual loss of synapses, which is consistent with the progress of cognitive dysfunction in AD. Similarly, there are studies showing that the sensitivity and specificity of NAA/Cr of PC in predicting the conversion of MCI to AD were 82% and 72%, respectively (Fayed et al., 2008). Consequently, NAA may be considered as a biomarker for monitoring the progression of MCI to AD. mI is a good indicator of the proliferation of glial cells, as it is mainly expressed in glial cells. Previous studies have reported the increased levels of mI and mI/Cr in the PC were found in MCI and AD patients (Yang et al., 2012), which was consistent with our research results. In this meta-analysis, the results showed that in the pathological development from MCI to AD, in addition to the decrease in the ratios of NAA/Cr and the concentration of NAA in PC, there was also an increase in the ratios of mI/Cr and mI/NAA; and, compared with the HC, the mI concentration in the PC of AD and MCI patients was significantly increased. Meanwhile, the level of mI in MCI and AD patients was significantly higher than that in the healthy subjects in the other brain regions, such as the temporo-parietal lobe and PWM. The increase in mI concentration may be caused by the activation of astrocytes or microglia, which is related to the neuroinflammation process, and has been considered as one of the core pathological features of AD. In addition, the increased mI may affect the phosphorylation of membrane proteins or cause changes in phospholipid metabolism, affecting the formation of Aβ, and thus leading to the deposition of amyloid plaques. In addition, the increased deposition of Aβ also induced the formation of nutritionally impaired synapses, and the astrocytes wrapped and phagocytosed the diseased synapses to remove the aberrations in the synapses. However, with the development of the disease, the deposition intensified, and this pathological change promoted the increase of inflammatory response, which would disrupt the normal form of synapses (Gomez-Arboledas et al., 2018). Studies have found that the change in the ratio of mI/Cr in PC was closely related to the early decline of cerebrospinal fluid Aβ42, and the decrease in the level of CSF Aβ42 can be detected 10-20 years before the onset of cognitive impairment (Bateman et al., 2012). Studies have found in the brain of Down's syndrome and other dementia patients, the ratio of mI/Cr was also significantly increased before significant manifestations of cognitive dysfunction (Voevodskaya et al., 2016). Therefore, these results suggested that the change in the level of mI may precede the onset of cognitive impairment, which had the potential to be applied to early diagnosis of AD.
Moreover, it is worth noting that this study showed that the concentration of Cr was significantly lower in the hippocampus of AD and MCI patients than that of healthy people, but there was no significant change in other brain regions such as PC. Interestingly, it is generally believed that the concentration of Cr is basically constant and uniformly distributed throughout the brain and is not changed with age or various diseases. Therefore, the level of Cr is often used as a reference value to indicate the level of other neurochemical substances. Some studies have also found that the concentration of Cr was relatively reduced in the late stage of AD as well as subcortical ischemic vascular dementia, which may be due to the fact that Cr existed in neurons and glial cells at the same time, and was affected by the density of brain tissue (Shiino et al., 2012).
Studies have regarded the ratio of mI/NAA a standard method to determine the severity of AD, as the sensitivity and specificity of the ratio of mI/NAA in the diagnosis for AD patients were 83 and 98%, respectively, and was consistent with the MMSE score (Shiino et al., 2012). This meta-analysis found that the ratio of mI/NAA increased during the progression from MCI to AD in PC, and the same result was also observed in the hippocampus. But since only 3 studies were included and a large heterogeneity was observed, this result should be interpreted cautiously. In addition, there was no meta-analysis results in the procession from MCI to AD in the hippocampus. Therefore, the change of mI/NAA is consistent in the progression from MCI to AD, but whether it can be used as markers in early diagnosis of AD is still questionable.

Different Metabolic Changes of Neurochemicals Between MCI and AD
The findings with respect to changes in the levels of Cho and Glx in MCI and AD were less consistent. As discussed above, the ratio of Cho/Cr was significantly higher in the PC of AD patients than that of HC, but there was a downward trend in MCI subjects. In addition, the concentration of Cho was found to be raised in the PC of AD patients compared with healthy controls, and no differences were seen in the MCI subjects. In contrast, the concentration of Cho was found to be reduced in the hippocampus of AD patients and MCI patients compared with healthy controls. It had been reported that a cholinergic lesion emerged as early as the MCI state and primarily in the presynaptic membrane, which may affect the long-term potentiation (Nordberg and Winblad, 1986). Cho plays an important role in the formation of cell membranes, and the change of concentration directly reflects the synthesis and degradation of membranes. Cho could be converted into acetylcholine (AchE) by choline acetyltransferase (ChAT) to play a neuroregulatory role, and the Cho signal may be closely related to the activity of ChAT (Klein, 2000). A clinical autopsy study found increased ChAT activity in the hippocampus of patients with MCI, which could explain that the decrease of Cho in the MCI stage is due to the increased activity of ChAT and the utilization of more choline substrates, thus resisting the damaging effect of cholinergic neurons (Ikonomovic et al., 2003). This compensatory activity may increase with the progression of the disease. Meanwhile, neuronal death will lead to an increase in membrane turnover, which will increase the ratio of Cho/Cr in AD patients. Recent investigations reported that the increase in the level of Cho in PC of AD patients may be the result of cell membrane rupture providing free Cho, which was in response to a decrease in the release of acetylcholine from cholinergic neurons in the brain of AD patients (Watanabe et al., 2010). In frontotemporal dementia and dementia with Lewy bodies, the increase in the ratio of Cho/Cr in the PC can also be detected. Interestingly, there was no significant change in the early stage of AD disease. In MCI patients, the concentration of Cho in PC remained basically the same with healthy controls, while the ratio of Cho/Cr had an upward trend, which may be due to the gradual aggravation of cholinergic neuron damage with the progression of the disease. Currently, cholinergic inhibitors such as donepezil, rivastigmine, and galanthamine are clinically used to treat AD. A meta-analysis showed that these drugs had modest but clinically significant overall benefits in stabilizing cognition, function, behavior, and overall clinical changes (Tan et al., 2014). Therefore, the change of Cho may reflect the severity of AD and was considered as a potential target for early detection and interventions.
In recent years, more research has focused on the change of Glx in patients with MCI and AD. Glx is a class of excitatory amino acid, including glutamate (Glu) and glutamine (Gln) (Bleich et al., 2003). In the brain, Glu and Gln are in dynamic equilibrium, and they can maintain and regulate synaptic information transmission through mutual transformation. In addition, Glu plays a crucial role in mitochondrial metabolism, neurotransmission of pyramidal cells, cerebral cortex function, and glutamate/GABA-glutamine cycle. And Glu-mediated synaptic transmission is critical for brain functions. However, excessive and continuous excitatory glutamatergic stimulation can lead to the death of neurons . Interestingly, our results showed that compared with HC, the concentrations of Glx and Glu in PC of AD patients were lower, while the ratio of Glx/Cr was relatively higher, and the ratio of Glx/Cr in the PC of MCI patients was higher, while the concentration of Glu had a downward trend. In animal experiments, it was also found that the Glu/Cr was decreased in AD model mice (Liang et al., 2017). Studies have reported that the Aβ can induce several changes in nerve cells including the loss of neuronal viability and synaptic activity, leading to the reduction in glutamate levels. Meanwhile, the decrease of Glu content will affect the Aamino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), which play a key role in synaptic function and cognition. In addition, this decrease in AMPARs may be the reason for the loss of synaptic and the decrease of cognitive function in AD (Liu et al., 2010). Consequently, Glx/Cr and Glu may be seen as signs of cognitive deterioration in AD.

Limitations
Several limitations to the current meta-analysis should be pointed out. First, the number of longitudinal studies to investigate the changes of metabolites between MCI-converter and MCI-stable patients was limited, so the sample size for analysis was relatively small. Therefore, more longitudinal studies are required to observe and explain the metabolite changes during the progress of MCI to AD. In addition, a significant effect of heterogeneity was found in many studies, and we were temporarily unable to do any moderating analysis to detect systematic influence on heterogeneity. Additionally, the detection results of MRS are affected by multiple parameters such as TR, TE, and ROI. This meta-analysis did not unify these parameters, which may lead to heterogeneity and affect the results.

CONCLUSION
In conclusion, the main findings of our meta-analysis revealed robust metabolite changes in the PC and the hippocampus during the development from MCI to AD, especially the levels of NAA and mI show high accuracy in the discrimination between healthy controls, MCI, and AD, but were also able to predict the possible progression from MCI to AD.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

AUTHOR CONTRIBUTIONS
SL, SH, and HLiu designed the study. SL and HLin revised the manuscript. HLiu, DZ, and HLin wrote the initial manuscript. HLiu, DZ, LZ, YZ, XY, and ZL collected the data and undertook the statistical analysis. SL and QZ critically reviewed and modified the paper. All authors contributed to the article and approved the submitted version.